Native coals are a mixture of carbon, hydrocarbons, moisture and polluting minerals with higher atomic numbers. Coal generates half of the United States electricity, but utilities face pressure to reduce their carbon footprint and the contamination from mercury, sulfur and coal ash. It is very expensive for the utilities to cleanup ash spills and to provide necessary pollution controls. The United States Environmental Protection Agency is now requiring stricter controls on the emission of mercury and sulfur. Further, new regulations will be imposing an hourly limit on sulfur emissions, rather than an average over twenty four hours. Generally, 60% to 80% of the mercury is associated with the sulfur in iron pyrites. The typical natural content of pollutants in coal used in the U.S. ranges from about 3% percent to 30% with an average of about 10% depending upon the region from which the coal was mined.
The combustion of coal in utility and industrial boilers generates millions of tons of coal ash, slag and sludge. Combustion removes burnable organic constituents but concentrates the naturally occurring radionuclides, which includes uranium, radium thorium and potassium in the ash. Coal ash also contains silicon, aluminum, iron, and calcium. In fact, these elements make up about 90% of the constituents of coal ash. Reduction in mercury emissions are needed to comply with Environmental Protection Agency regulations. Options to reduce mercury emissions include selective mining of coal (avoiding parts of a coal bed that are higher in sulfur and mercury), coal washing (to remove iron pyrite which contains 60% to 80% of the mercury in the coal), post-combustion removal of mercury from the stack emissions or the use of natural gas in place of coal.
Current coal processing uses the difference between the densities of coal and contaminants to remove non-combustibles. Some 95% of coal processing currently uses wet methods. Coal typically has a specific gravity of 1.2 while the rock and heavier minerals have average values of 2.5. Run of the mine coal is typically first reduced to sizes under two inches (5 cm) before it is introduced into a water-magnetite slurry flotation media. The said water slurry has chemicals that raise the specific gravity of the liquid to a value above that of coal. The proportion of magnetite in the water slurry controls the density. The heavier sulfur and silicates sink while the lighter coal floats off.
Wet processing can reduce the ash and sulfur content of the coal, but it wets the processed coal. Furthermore, the liquid media requires treatment in a wastewater treatment facility. Coal fines and water produce sludge with environmental problems. Some processes use acids to remove contaminants and pollute water. The latent heat of water in wet coal reduces the recoverable energy from the combustion of coal by one to two percent. This reduction in useful energy increases the carbon footprint to produce electrical power.